Cellular radiocommunications systems are based on subdividing a geographical area into small portions or cells, each of which is served by one or more base stations. Such subdivision into cells makes it possible to optimize use of the radio spectrum since a plurality of cells can use the same portion of the spectrum.
Each base station can communicate via radio links with a plurality of mobile stations travelling in its cell. Furthermore, the base stations are connected to a switching center or mobile switching center (MSC) which establishes connection between a mobile station and a remote terminal (e.g. a subscriber of the switched network or another mobile station). The switching center and the base station are interconnected via a PCM (pulse coded modulation) line.
The cost of using a PCM line is very high. It is therefore desirable to optimize use of such a line. To this end, a transcoder (or TRAU) is generally inserted between the base stations and the switching center, so that data interchange (whether it be data or speech that is actually interchanged) between a base station and the transcoder is compressed.
Both the transcoder and the base station include compression and decompression means. For example, the data-rate of a call may be reduced from 64 kbit/s to 16 kbit/s in the GSM system ("groupe special mobile" or global system for mobile communications) to which the invention advantageously applies. The improvement obtained in such a system is therefore by a factor of 4.
In order to achieve an optimum improvement, it is desirable for the compression to take place over as long a distance as possible. In this way, in general, the transcoder is installed directly on the same site as the switching center.
In any communications systems, transmission errors can appear, for many reasons (a line being cut off, external disturbance, breakdowns, etc.). The link between a transcoder and a base station is no exception to this rule. As a result, it is necessary to detect and manage such transmission errors.
In a GSM radiocommunications system, error management is performed as follows:
if the base station detects synchronization loss for one second, it transmits an alarm to a BSC (base station controller) that controls said base station, which BSC then decides to interrupt the call; and PA1 if the transcoder detects synchronization loss for one second, it stops transmitting so as to transmit an emergency alarm (in compliance with GSM Recommendation 08-60). On receiving the alarm, the base station warns the BSC that controls it, which BSC then decides to interrupt the call. PA1 a signal from a mobile station to a remote terminal (up direction) being conveyed: PA1 said transcoder performing the following operations: PA1 and said base station performing the following operations: PA1 counting the number of errors that appear during a third predetermined lapse of time; PA1 activating an emergency clock when said number of errors is greater than a predetermined threshold; and PA1 interrupting the call if said emergency clock reaches a predetermined time-delay value, and if said transcoder receives a call interruption message.
That method suffers from two major drawbacks.
Firstly, the transcoder has to interrupt the flow of data to indicate a transmission problem. Such an interruption can prove to be superfluous. It is not infrequent for a call to be disturbed in one transmission direction only. Therefore, just because the transcoder detects disturbances in the up direction (from the base station to the transcoder), it does not mean the link is disturbed in the down direction (from the transcoder to the base station). In which case, it is unnecessary and even undesirable to interrupt data transmission to the base station.
Secondly, it appears that the length of the time delay (1 second) before concluding that synchronization loss has occurred is too short for many operators (e.g. when the network used is of medium or low quality). It is therefore desirable for this value to be parameterizable.
This poses few problems for the base stations, in which the operating software is downloaded. However, the transcoder software is conventionally stored in a read-only memory, and is therefore not downloadable. Therefore, in order to modify the length of the time delay, it is necessary to change the memory of the transcoder, which is a costly operation.
Another possibility consists in providing a transcoder configuration message for modifying the length of the time delay. But, in addition to making it necessary to define a new message to the transcoder, that possibility makes it necessary for the length of the time delay to be identical regardless of the BTS to which the transcoder is connected. This gives rise to drawbacks when the quality of the PCM lines between a BSC and the various BTSs that it controls is not constant.